Van Ryt, Matthew, Sanislav, Ioan, Dirks, Paul, Huizenga, Jan, Mturi, Marwa, and Kolling, Sergio (2019) Biotite chemistry and the role of halogens in Archaean greenstone hosted gold deposits: a case study from Geita gold mine, Tanzania. Ore Geology Reviews, 111. 102982.

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Abstract

Geita Hill is one of the largest Tanzanian gold deposits, located southwest of Lake Victoria and ∼2 km along strike from the giant Nyankanga deposit. The deposit is hosted by metasedimentary rocks – dominated by ironstones and intruded by numerous diorite dykes. Textural, structural and geochronological observations indicate that the gold mineralisation overprints all the ductile fabrics and the late orogenic D6 reverse faulting event, suggesting that gold was introduced during later D7 and D8 events. The metamorphic background is characterized by biotite+K-feldspar+actinolite ± pyrhotite, which is overprinted by the gold related alteration consisting of sulfidation fronts, veins and micro-fractures dominated by quartz+biotite+Kfeldpar+pyrite. The almost identical mineralogy of the mineralised assemblage and the metamorphic background makes it difficult to identify the source, the effect and the spatial extent of the mineralising fluid. As part of this study detailed analyses of biotite were conducted in order to identify the nature of the mineralising fluid and its spatial effect across the Geita Hill deposit. Results show that the mineralised assemblage is slightly more oxidized (pyrite+magnetite) compared to the metamorphic background pyrrhotite+magnetite). The intense sulfidation within the ore zone resulted in the formation of Mg-rich biotite, which grades into more Fe-rich biotite away from the ore zone. This change in biotite chemistry causes a shift in the SWIR Fe-OH 2250 nm absorption feature to lower wavelengths, which is also correlated with an increase in gold grade within the mineralised zone. The abundance of F in biotite significantly increases within the ore zone indicating the presence of F-rich hydrothermal fluids during mineralisation. Late timing of mineralisation, synchronous with the emplacement of voluminous high-K granites, and the presence of K- and F-bearing fluids indicate a genetic connection between the mineralising fluids and the late granites. Biotite grains from different deposits types (porphyry, IOCG, lode-Au) appear to have distinct composition suggesting that the source of the fluids involved in the formation of each deposit type has a characteristic halogen chemistry.

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